(1) Field of the Invention
The present invention relates to an apparatus for feedback control of the air-fuel ratio in an internal combustion engine.
(2) Description of the Related Art
Generally, in an air-fuel ratio feedback control system of an internal combustion engine, a base fuel amount is calculated in accordance with the intake air amount (or the intake air pressure) and the engine speed, and is corrected by using an air-fuel ratio correction amount calculated in accordance with a detection signal of an air-fuel ratio sensor (so-called 0.sub.2 sensor) which detects the concentration of a specific component, such as oxygen, in the exhaust gas. Thus, the corrected fuel amount determines an actual fuel amount to be supplied to the engine. The above-mentioned steps are repeated to control the center value of the controlled air-fuel ratio within a very narrow range around the stoichimetric ratio required for three-way reducing and oxidizing catalysts.
In the prior art, when the condition of the air-fuel ratio sensor is completely deteriorated and it no longer functions correctly, the feedback control of the air-fuel ratio is stopped and an alarm is generated.
Recently, engines with a high fuel efficiency and a low friction coefficient have been developed. In such engines, the temperature of the exhaust gas may be reduced, thus cooling the air-fuel ratio sensor. In this case, when the condition of the air-fuel ratio sensor is completely deteriorated and it no longer functions correctly, the feedback control of the air-fuel ratio is stopped. However, when the condition of the air-fuel ratio sensor is not fully deteriorated, as a result of individual differences in and aging thereof, the air-fuel ratio sensor still exhibits a partial functioning. As a result, the feedback control of the air-fuel ratio may be carried out in such a manner that the controlled center of the air-fuel ratio often deviates from an optimum value, thus increasing the exhaust emission such as NO.sub.x, reducing the drivability, and reducing the fuel efficiency.